U.S. patent number 6,825,480 [Application Number 09/603,459] was granted by the patent office on 2004-11-30 for charged particle beam apparatus and automatic astigmatism adjustment method.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Yasuhiro Gunji, Kouichi Hayakawa, Masami Iizuka, Hiroyuki Shinada, Atsuko Takafuji, Masayoshi Takeda, Masahiro Watanabe.
United States Patent |
6,825,480 |
Watanabe , et al. |
November 30, 2004 |
Charged particle beam apparatus and automatic astigmatism
adjustment method
Abstract
According to the invention, techniques for automatically
adjusting for astigmatism in a charged particle beam apparatus.
Embodiments according to the present invention can provide a
charged particle beam apparatus and an automatic astigmatism
adjustment methods capable of automatically correcting astigmatism
and a focal point in a relatively short period of time by finding a
plurality of astigmatism correction quantities and a focal point
correction quantity in a single operation from a relatively small
number of 2 dimensional images. Specific embodiments can perform
such automatic focusing while minimizing damages inflicted on
subject samples. Embodiments include, among others, a charged
particle optical system for carrying out an inspection, a
measurement and a fabrication with a relatively high degree of
accuracy by using a charged particle beam.
Inventors: |
Watanabe; Masahiro (Yokohama,
JP), Shinada; Hiroyuki (Chofu, JP),
Takafuji; Atsuko (Tokyo, JP), Iizuka; Masami
(Ishioka, JP), Gunji; Yasuhiro (Hitachioota,
JP), Hayakawa; Kouichi (Hitachinaka, JP),
Takeda; Masayoshi (Hitachinaka, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
33446906 |
Appl.
No.: |
09/603,459 |
Filed: |
June 22, 2000 |
Foreign Application Priority Data
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Jun 23, 1999 [JP] |
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P11-176681 |
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Current U.S.
Class: |
250/491.1;
250/310; 250/311; 250/396R; 250/398; 250/492.1; 250/492.2;
250/492.21; 356/401; 430/30 |
Current CPC
Class: |
G01R
31/307 (20130101); H01J 37/153 (20130101); H01J
2237/2817 (20130101); H01J 2237/216 (20130101); H01J
2237/1532 (20130101) |
Current International
Class: |
G01R
31/28 (20060101); G01R 31/307 (20060101); G01B
011/00 () |
Field of
Search: |
;250/306,310,311,396R,396ML,491.1,492.1,492.2,492.21,492.22,397,398
;356/401 ;430/30 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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09082257 |
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Mar 1997 |
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JP |
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09161706 |
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Jun 1997 |
|
JP |
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10106469 |
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Apr 1998 |
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JP |
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Primary Examiner: Lee; John R.
Assistant Examiner: Vanore; David A.
Attorney, Agent or Firm: Townsend and Townsend and Crew
LLP
Claims
What is claimed is:
1. A charged particle beam apparatus comprising: a stage for
setting a sample; a charged particle optical system for converting
a charged particle beam emitted by a charged particle source; a
scanning unit for irradiating said charged particle beam converged
by said charged particle optical system to said sample in order to
scan said sample; a focal position control system for controlling a
focal position of said charged particle beam converged by said
charged particle optical system; an astigmatism adjustment unit for
adjusting astigmatism of said charged particle beam converged by
said charged particle optical system; a particle image detection
unit for obtaining a plurality of 2-dimensional particle images by
detection of a particle images generated by said sample scanned by
the irradiation of said charged particle beam converged by said
charged particle optical system, where a single 2-dimensional
particle image is obtained for each focal position; an image
processing unit for computing a focal offset and said astigmatism
of said converged charged particle beam on the basis of said
plurality of 2-dimensional particle images obtained by said
particle image detection unit at different focal positions
controlled by said focal position control system; and a control
system for adjusting and controlling said astigmatism of said
converged charged particle beam by feeding back an astigmatism
correction quantity based on said astigmatism computed by said
image processing unit to said astigmatism adjustment, wherein a
cross-sectional shape of said charged particle beam at an
astigmatism adjusted focal position is circle and said image
processing means computes said astigmatism by using at least three
directional sharpness magnitudes which are obtained from said
single 2-dimensional particle image at each focal position, wherein
said charged particle beam apparatus creates a pattern, said
pattern including edge components in at least 3 directions on said
sample, said pattern having at least 3 areas, each of said areas
for creating a sub pattern having one of said edge components in
one of said directions on said sample.
2. A charged particle beam apparatus comprising: a stage for
setting a sample; a charged particle optical system for converging
a charged particle beam emitted by a charged particle source; a
scanning unit for irradiating said charged particle beam converged
by said charged particle optical system to said sample in order to
scan said sample; a focal position control system for controlling a
focal position of said charged particle beam converged by said
charged particle optical system; an astigmatism adjustment unit for
adjusting astigmatism of said charged particle beam converged by
said charged particle optical system; a particle image detection
unit for obtaining a plurality of 2-dimensional particle images by
detection of a particle images generated by said sample scanned by
the irradiation of said charged particle beam converged by said
charged particle optical system, where a single 2-dimensional
particle image is obtained for each focal position; an image
processing unit for computing a focal offset and said astigmatism
of said converged charged particle beam on the basis of said
plurality of 2-dimensional particle images obtained by said
particle image detection unit at different focal positions
controlled by said focal position control system; and a control
system for adjusting and controlling said astigmatism of said
converged charged particle beam by feeding back an astigmatism
correction quantity based on said astigmatism computed by said
image processing unit to said astigmatism adjustment, wherein a
cross-sectional shape of said charged particle beam at an
astigmatism adjusted focal position is circle and said image
processing means computes said astigmatism by using at least three
directional sharpness magnitudes which are obtained from said
single 2-dimensional particle image at each focal position, and
wherein said particle image detection unit detects a particle image
generated from said sample serving as an object substrate as a
result of radiation of said converged charged particle beam with at
least said astigmatism adjusted and controlled by said control
system to said object substrate in a scanning operation carried out
by using said scanning unit; and a defect inspection image
processing unit is provided for inspecting said object substrate
for a defect existing on said object substrate on the basis of said
detected particle image.
3. A charged particle beam apparatus comprising: a stage for
setting a sample; a charged particle optical system for converging
a charged particle beam emitted by a charged particle source; a
scanning unit for irradiating said charged particle beam converged
by said charged particle optical system to said sample in order to
scan said sample; a focal position control system for controlling a
focal position of said charged particle beam converged by said
charged particle optical system; an astigmatism adjustment unit for
adjusting astigmatism of said charged particle beam converged by
said charged particle optical system; a particle image detection
unit for obtaining a plurality of 2-dimensional particle images by
detection of a particle images generated by said sample scanned by
the irradiation of said charged particle beam converged by said
charged particle optical system, where a simple 2-dimensional
particle image is obtained for each focal position; an image
processing unit for computing a focal offset and said astigmatism
of said converged charged particle beam on the basis of said
plurality of 2-dimensional particle images obtained by said
particle image detection unit at different focal positions
controlled by said focal position control system; and a control
system for adjusting and controlling said astigmatism of said
converged charged particle beam by feeding back an astigmatism
correction quantity based on said astigmatism computed by said
image processing unit to said astigmatism adjustment, wherein a
cross-sectional shape of said charged particle beam at an
astigmatism adjusted focal position is circle and said image
processing means computes said astigmatism by using at least three
directional sharpness magnitudes which are obtained from said
single 2-dimensional particle image at each focal position, wherein
said particle image detection unit detects a particle image
generated from said sample serving as an object substrate as a
result of irradiation of said converged charged particle beam with
at least said astigmatism adjusted and controlled by said control
system to said object substrate in a scanning operation carried out
by using said scanning unit, and a measurement image processing
unit is provided for measuring dimensions of a pattern existing on
said object substrate on the basis of said detected particle image,
and wherein control of said focal position control system is based
on a height on said object substrate optically detected by a height
detection sensor further provided for optically detecting a height
on said object substrate.
4. A charged particle beam apparatus comprising: a stage for
setting a sample; a charged particle optical system for converging
a charged particle beam emitted by a charged particle source; a
scanning means for irradiating and scanning said charged particle
beam converged by said charged particle optical system on a surface
of said sample; a focal position control system for controlling a
focal position of said charged particle beam converged by said
charged particle optical system; an astigmatism adjustment means
for adjusting astigmatism of said charged particle beam converged
by said charged particle optical system; a particle image detection
means for obtaining a single 2-dimensional particle image at each
focal position by changing focal position with use of said focal
position control system and detecting particles generated from a
surface of said sample by the irradiation and the scanning of said
charged particle beam with use of said scanning means; an image
processing means for computing said astigmatism of said converged
charged particle beam on the basis of said 2 dimensional particle
images at each focal position obtained by said particle image
detection means; and a control system for adjusting and controlling
said astigmatism of said converged charged particle beam by feeding
back an astigmatism correction quantity based on said astigmatism
computed by said image processing means to said astigmatism
adjustment means, wherein said image processing means computes said
astigmatism of said converged charged particle beam from a relation
among in focus positions at directional sharpness magnitudes for at
least 3 directions by finding said directional sharpness magnitudes
for at least said 3 directions for a plurality of focal position
positions from said 2 dimensional particle image with a plurality
of focal position positions obtained by said particle image
detection means and then finding said in focus positions at said
found directional sharpness magnitudes for at least said 3
directions, and wherein said control system for adjusting and
controlling further provides adjusting and controlling of said
focal position of said converged charged particle beam by feeding
back a focal position correction quantity based on said focal
offset computed by said image processing means to said focal
position control system.
5. A charged particle beam apparatus comprising: a stage for
setting a sample; a charged particle optical system for converging
a charged particle beam emitted by a charged particle source; a
scanning means for irradiating and scanning said charged particle
beam converged by said charged particle optical system on a surface
of said sample; a focal position control system for controlling a
focal position of said charged particle beam converged by said
charged particle optical system; an astigmatism adjustment means
for adjusting astigmatism of said charged particle beam converged
by said charged particle optical system; a particle image detection
means for obtaining a single 2-dimensional particle image at each
focal position by changing focal position with use of said focal
position control system and detecting particles generated from a
surface of said sample by the irradiation and the scanning of said
charged particle beam with use of said scanning means; an image
processing means for computing said astigmatism of said converged
charged particle beam on the basis of said 2 dimensional particle
images at each focal position obtained by said particle image
detection means; and a control system for adjusting and controlling
said astigmatism of said converged charged particle beam by feeding
back an astigmatism correction quantity based on said astigmatism
computed by said image processing means to said astigmatism
adjustment means, wherein said charged particle beam apparatus is
characterized in that said particle image detection means has a
configuration wherein a particle image having a plurality of
different focal positions is detected from a plurality of different
areas on said sample, and wherein said sample is inclined or has a
staircase like surface.
6. A charged particle beam apparatus comprising: a stage for
setting a sample; a charged particle optical system for converging
a charged particle beam emitted by a charged particle source; a
scanning means for irradiating and scanning said charged particle
beam converged by said charged particle optical system on a surface
of said sample; a focal position control system for controlling a
focal position of said charged particle beam converged by said
charged particle optical system; an astigmatism adjustment means
for adjusting astigmatism of said charged particle beam converged
by said charged particle optical system; a particle image detection
means for obtaining a single 2-dimensional particle image at each
focal position by changing focal position with use of said focal
position control system and detecting particles generated from a
surface of said sample by the irradiation and the scanning of said
charged particle beam with use of said scanning means; an image
processing means for computing said astigmatism of said converged
charged particle beam on the basis of said 2 dimensional particle
images at each focal position obtained by said particle image
detection means; and a control system for adjusting and controlling
said astigmatism of said converged charged particle beam by feeding
back an astigmatism correction quantity based on said astigmatism
computed by said image processing means to said astigmatism
adjustment means, wherein said image processing means computes said
astigmatism of said converged charged particle beam from a relation
among in focus positions at directional sharpness magnitudes for at
least 3 directions by finding said directional sharpness magnitudes
for at least said 3 directions for a plurality of focal position
positions from said 2 dimensional particle image with a plurality
of focal position positions obtained by said particle image
detection means and then finding said in focus positions at said
found directional sharpness magnitudes for at least said 3
directions, wherein said image processing means determines an in
focus position for each of directional sharpness magnitudes as a
center of gravity of an area enclosed by a segment of a curve and a
horizontal line representing a threshold value where said curve
represents variations of each of said directional sharpness
magnitudes with respect to said in focus position whereas said
segment represents said variations exceeding said threshold
value.
7. A charged particle beam apparatus comprising: a stage for
setting a sample; a charged particle optical system for converging
a charged particle beam emitted by a charged particle source; a
scanning means for irradiating and scanning said charged particle
beam converged by said charged particle optical system on a surface
of said sample; a focal position control system for controlling a
focal position of said charged particle beam converged by said
charged particle optical system; an astigmatism adjustment means
for adjusting astigmatism of said charged particle beam converged
by said charged particle optical system; a particle image detection
means for obtaining a single 2-dimensional particle image at each
focal position by changing focal position with use of said focal
position control system and detecting particles generated from a
surface of said sample by the irradiation and the scanning of said
charged particle beam with use of said scanning means; an image
processing means for computing said astigmatism of said converged
charged particle beam on the basis of said 2 dimensional particle
images at each focal position obtained by said particle image
detection means; and a control system for adjusting and controlling
said astigmatism of said converged charged particle beam by feeding
back an astigmatism correction quantity based on said astigmatism
computed by said image processing means to said astigmatism
adjustment means, wherein said image processing means computes said
astigmatism of said converged charged particle beam from a relation
among in focus positions at directional sharpness magnitudes for at
least 3 directions by finding said directional sharpness magnitudes
for at least said 3 directions for a plurality of focal position
positions from said 2 dimensional particle image with a plurality
of focal position positions obtained by said particle image
detection means and then finding said in focus positions at said
found directional sharpness magnitudes for at least said 3
directions, wherein said image processing means determines said in
focus position for each of directional sharpness magnitudes by:
computing a degree of matching between a curve representing
variations of an evaluation value with respect to each of said
directional sharpness magnitudes and any one of curves of image
inversion which are each symmetrical with respect to an axis of
symmetry on the right and left sides of said axis of symmetry;
determining a specific one of said curves of image inversion with a
highest degree of matching; and using the position of an axis of
symmetry of said specific curve of image inversion as said in focus
position.
8. A charged particle beam apparatus comprising: a stage for
setting a sample; a charged particle optical system for conversing
a charged particle beam emitted by a charged particle source; a
scanning means for irradiating and scanning said charged particle
beam converged by said charged particle optical system on a surface
of said sample; a focal position control system for controlling a
focal position of said charged particle beam converged by said
charged particle optical system; an astigmatism adjustment means
for adjusting astigmatism of said charged particle beam converged
by said charged particle optical system; a particle image detection
means for obtaining a single 2-dimensional particle image at each
focal position by changing focal position with use of said focal
position control system and detecting particles generated from a
surface of said sample by the irradiation and the scanning of said
charged particle beam with use of said scanning means; an image
processing means for computing said astigmatism of said converged
charged particle beam on the basis of said 2 dimensional particle
images at each focal position obtained by said particle image
detection means; and a control system for adjusting and controlling
said astigmatism of said converged charged particle beam by feeding
back an astigmatism correction quantity based on said astigmatism
computed by said image processing means to said astigmatism
adjustment means, wherein said image processing means computes said
astigmatism of said converged charged particle beam from a relation
among in focus positions at directional sharpness magnitudes for at
least 3 directions by finding said directional sharpness magnitudes
for at least said 3 directions for a plurality of focal position
positions from said 2 dimensional particle image with a plurality
of focal position positions obtained by said particle image
detection means and then finding said in focus positions at said
found directional sharpness magnitudes for at least said 3
directions, wherein said charged particle beam apparatus further
comprises: a standard sample is provided for calibration purposes
at a location adjacent to an object substrate; at least astigmatism
or a focal position is corrected on said standard sample prior to
an observation, an inspection or a measurement of said object
substrate or periodically; wherein said observation, said
inspection or said measurement of said object substrate is carried
out in a state of corrected astigmatism or a corrected focal
position.
9. An automatic astigmatism adjustment method comprising:
converging a charged particle beam emitted from a charged particle
source; irradiating said converged charged particle beam to a
sample with a pattern formed thereon; obtaining a plurality of 2
dimensional particle images having different focal positions of
said converged particle beam by detection of particles generated
from said sample by said radiating; computing directional sharpness
magnitudes for at least 3 directions for a plurality of focal
position positions from said plurality of 2 dimensional particle
images; computing in focus positions using said computed
directional sharpness magnitudes for at least said 3 directions;
computing astigmatism of said converged charged particle beam from
a relation among said computed in focus positions at said computed
directional sharpness magnitudes for at least said 3 directions;
and controlling said astigmatism of said converged charged particle
beam by feeding back an astigmatism correction quantity computed
based on said astigmatism; and said focal position of said
converged charged particle beam by feeding back a focal position
correction quantity computed based on said in focus positions,
wherein said computing an in focus position using said computed
directional sharpness magnitudes further comprises: computing a
center of gravity of an area enclosed by a segment of a curve and a
horizontal line representing a threshold value where said curve
represents variations of each of said computed directional
sharpness magnitudes with respect to said in focus position; and
wherein said segment represents said variations exceeding said
threshold value.
10. An automatic astigmatism adjustment method comprising:
converging a charged particle beam emitted from a charged particle
source; irradiating said converged charged particle beam to a
sample with a pattern formed thereon; obtaining a plurality of 2
dimensional particle images having different focal positions of
said conversed particle beam by detection of particles generated
from said sample by said radiating; computing directional sharpness
magnitudes for at least 3 directions for a plurality of focal
position positions from said plurality of 2 dimensional particle
images; computing in focus positions using said computed
directional sharpness magnitudes for at least said 3 directions;
computing astigmatism of said converged charged particle beam from
a relation among said computed in focus positions at said computed
directional sharpness magnitudes for at least said 3 directions;
and controlling said astigmatism of said converged charged particle
beam by feeding back an astigmatism correction quantity computed
based on said astigmatism; and said focal position of said
converged charged particle beam by feeding back a focal position
correction quantity computed based on said in focus positions,
wherein said computing an in focus position using said computed
directional sharpness magnitudes further comprises: computing a
degree of matching between a curve representing variations of an
evaluation value with respect to each of said directional sharpness
magnitudes and any one of curves of image inversion which are each
symmetrical with respect to an axis of symmetry on the right and
left sides of said axis of symmetry; determining a specific one of
said curves of image inversion with a highest degree of matching;
and using the position of an axis of symmetry of said specific
curve of image inversion as said in focus position.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims priority from Japanese Patent Application
Reference No. 11-176681, filed Jun. 23, 1999.
BACKGROUND OF THE INVENTION
The present invention relates generally to a charged particle beam
apparatus and specifically to techniques for automatically
adjusting for astigmatism.
An electron microscope is used as an automatic inspection system
for inspecting and measuring a fine circuit pattern created on a
substrate such as a semiconductor wafer. In the case of a defect
inspection, an electron beam image detected by a scanning electron
microscope is acquired and compared with a reference image used as
a comparison standard. In a measurement of a hole diameter or a
fine circuit pattern used in monitoring and setting a manufacturing
process condition of a semiconductor device, measurement of a
length is based on image processing of an electron beam image
detected from the scanning electron beam microscope. In an
inspection to detect a defect of a pattern by comparison of an
electron beam image of the pattern with a reference image and in a
measurement of a line width of a pattern by processing an electron
beam image, the quality of the electron beam image greatly affects
the reliability of the result of the inspection and the
measurement.
The quality of an electron beam image deteriorates due to causes
such as astigmatism of the electron beam system and degradation of
the resolution attributed to defocusing. A poor quality electron
beam image causes the inspection sensitivity and the performance of
the length measurement to deteriorate. In addition, in such an
image, the pattern width varies and a result of detection of an
image edge can not be obtained in a relatively stable manner. In
consequence, results of measuring the wire width of a pattern and
the diameter of a hole with such a poor quality beam will often be
unsatisfactory.
Conventionally, the focal point and the astigmatism of an electron
beam optical system are adjusted by properly changing a control
current of an objective lens and control currents of 2 coil sets
each comprising a plurality of astigmatism correction coils while
visually observing an electron beam image. To be more specific, the
focal point is adjusted by varying the convergence height of a
beam. The convergence height of a beam is changed by adjusting a
current flowing through the objective lens.
While there are perceived advantages, it can take time to execute
the conventional technique of adjusting a control current of an
objective lens and control currents of 2 coil sets each comprising
a plurality of astigmatism correction coils while visually
observing an electron beam image as described above. In addition,
the conventional techniques often require that the surface of a
sample be scanned by using an electron beam several times. As a
result, it is quite within the bounds of possibility that a problem
of a damage inflicted on the sample arises. In addition, since in
conventional systems, adjustments are often carried out manually,
the result of the adjustment varies from operator to operator.
Moreover, the astigmatism and the focal position can change with
time. It is thus necessary to adjust the astigmatism and the focal
position periodically by manual operations in order to carry out an
automatic inspection and an automatic measurement of a length.
What is needed are automated techniques for controlling electron
beams.
SUMMARY OF THE INVENTION
According to the invention, techniques for automatically adjusting
for astigmatism in a charged particle beam apparatus are provided.
Embodiments according to the present invention can provide a
charged particle beam apparatus and an automatic astigmatism
adjustment methods capable of automatically correcting astigmatism
and a focal point in a relatively short period of time by finding a
plurality of astigmatism correction quantities and a focal point
correction quantity in a single operation from a relatively small
number of 2 dimensional images. Specific embodiments can perform
such automatic focusing while minimizing damages inflicted on
subject samples. Embodiments include, among others, a charged
particle optical system for carrying out an inspection, a
measurement and a fabrication with a relatively high degree of
accuracy by using a charged particle beam.
Numerous benefits are achieved by way of the present invention over
conventional techniques. The present invention can provide specific
embodiments with the capability of automatically adjusting
astigmatism and a focal point at a relatively high speed and with a
relatively high degree of precision by using a small number of
particle images obtained as a result of radiation of a converged
charged particle beam to a sample in a scanning operation without
inflicting a damage on the sample. In addition, select embodiments
according to the present invention can also provide the capability
of carrying out an automatic inspection of a pattern defect, such
as a foreign substance on an object substrate, or an automatic
measurement of dimensions of a pattern on the object substance. In
specific embodiments, such tasks can be carried out with a
relatively high degree of precision while sustaining the quality of
a particle image in a relatively stable manner over a relatively
long period of time by using the particle image. The particle image
is obtained as a result of radiation of a converged charged
particle beam while adjusting astigmatism and focal point thereof
automatically and at a relatively high speed and with a relatively
high degree of precision.
Select embodiments according to the present invention can provide a
charged particle beam apparatus and an automatic astigmatism
adjustment method capable of automatically correcting astigmatism
in a relatively short period of time for a variety of samples. Such
embodiments according to the invention can find a plurality of
astigmatism correction quantities at the same time from a small
number of 2 dimensional images. In specific embodiments, damage
inflicted on samples can be kept to a minimum.
Furthermore, some specific embodiments according to the present
invention can provide a charged particle beam apparatus capable of
carrying out inspections, measurements and fabrications with a
relatively high degree of reliability and in a relatively stable
manner over a relatively long period of time. The quality of a
charge article image obtained from an object substrate as a result
of automatic correction of stigmatism and a focal point of a
charged particle beam optical system is improved in some
embodiments.
Moreover, many specific embodiments according to the present
invention can provide a sample used for an adjustment of
astigmatism and a focal point of a charged particle beam and
suitable for an automatic correction of the astigmatism and the
focal point in a relatively short period of time by suppressing
damages inflicted on the sample to a minimum in a charged particle
beam optical system.
Yet further, some specific embodiments according to the present
invention can provide an automatic astigmatism adjustment method
capable of automatically correcting astigmatism and a focal point
in a relatively short period of time from a 2 dimensional particle
image and to provide a sample for the method.
These and other benefits are described throughout the present
specification. A further understanding of the nature and advantages
of the invention herein may be realized by reference to the
remaining portions of the specification and the attached
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a diagram showing a representative configuration
of an inspection and measurement apparatus in an example charged
particle beam apparatus provided by a particular embodiment of the
present invention;
FIG. 2 illustrates an explanatory diagram showing astigmatism
correction coils in a particular embodiment according to the
present invention;
FIG. 3 illustrates a diagram showing a relation of astigmatism and
shapes of a beam spot;
FIG. 4 illustrates a diagram showing representative patterns used
for correcting astigmatism and a focal point in a particular
embodiment of the present invention;
FIG. 5 illustrates a flowchart of representative image processing
executed by an astigmatism and focal point correction quantity
computation circuit in a particular embodiment according to the
present invention;
FIG. 6 illustrates diagram showing a relation among a computed
directional sharpness magnitude, an astigmatism magnitude, an
astigmatism direction and a focal offset z;
FIG. 7 illustrates a diagram showing representative image
processing in order to find directional sharpness magnitudes in a
particular embodiment according to the present invention;
FIG. 8 illustrates a diagram showing shapes of a calibration target
or a sample for calibration of astigmatism and a focal point at a
relatively high speed;
FIG. 9 illustrates a flowchart of representative image processing
executed by the astigmatism and focal point correction quantity
computation circuit employed in the charged particle beam apparatus
shown in FIG. 1 and using the calibration target shown in FIG. 8 in
a particular embodiment according to the present invention;
FIG. 10 illustrates a diagram showing a representative visual field
movement sequence for periodically calibrating drifts of
astigmatism and a focal point in a particular embodiment according
to the present invention;
FIG. 11 illustrates a diagram showing a technique of interpolation
of a maximum value position of a curve representing the directional
sharpness magnitude in a particular embodiment according to the
present invention;
FIG. 12 illustrates an explanatory diagram showing a case in which
a curve representing the directional sharpness magnitude exhibits a
double peaked characteristic in a particular embodiment according
to the present invention;
FIG. 13 illustrates a diagram showing a technique of finding a
center of gravity as a center position of a curve representing the
directional sharpness magnitude in a particular embodiment
according to the present invention;
FIG. 14 illustrates a diagram showing a technique of computing a
weighted average of positions of maximum values as a center
position of a curve representing the directional sharpness
magnitude in a particular embodiment according to the present
invention; and
FIG. 15 illustrates a diagram showing a technique of finding a
center position of a curve representing the directional sharpness
magnitude by symmetry matching in a particular embodiment according
to the present invention.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
The present invention provides techniques for automatically
adjusting for astigmatism in charged particle beam apparatus.
Embodiments according to the present invention can provide a
charged particle beam apparatus and automatic astigmatism
adjustment techniques capable of automatically correcting
astigmatism, as well as focal point, in a relatively short period
of time. Embodiments can find a plurality of astigmatism correction
quantities and a focal point correction quantity in a single
operation from a relatively small number of 2 dimensional
images.
A variety of automatic astigmatism correction techniques have been
proposed. According to one technique, a secondary electron signal
obtained from a sample as a result of 2 dimensional scanning over
the sample by using a charged particle beam is differentiated to
extract digital data with a large variation. Then, a position on
the sample corresponding to the extracted digital data is found.
The charged particle beam is then used for scanning the sample only
in the X and Y directions with the found position taken as a center
while an excitation current flowing through the an objective lens
is being changed. Then, focal information in the X direction and
focal information in the Y direction are detected at a maximum
value of the digital data of the secondary electron signal obtained
as a result of the scanning in each of these directions. From the
focal information in the X direction and the focal information in
the Y direction, the magnitude of a current flowing through the
objective lens is determined and the current of the determined
magnitude is output to the objective lens. Later, a current flowing
through an astigmatism correction coil is changed and a charged
particle beam is used for scanning in one direction, namely, the X
or Y direction, and the magnitude of the current flowing through
the astigmatism correction coil is determined at a maximum value of
the digital data of a secondary electron signal obtained as a
result of the scanning to adjust the focus and the astigmatism of
the output charged particle beam. For a more detailed description
of such techniques, reference may be had to a Japanese Patent
Prepublication No. Hei 7-153407.
According to another technique, while an electron beam is being
used in scanning in a variety of directions, a focal point is
vibrated to find the direction of astigmatism. Then, while a
relation between 2 astigmatism quantities of different types is
being sustained so that the astigmatism changes only in a
particular direction, the astigmatism quantities are changed to
search for a condition producing a clear image. Thus, conditions of
the astigmatism quantities of 2 freedoms can be adjusted by
limiting the quantities to 1 freedom, allowing the adjustment time
to be shortened. For a more detailed description of such
techniques, reference may be had to a Japanese Patent
Prepublication No. Hei 9-161706.
In a yet further technique, after a focal point is adjusted
automatically to make a shift from an in focus state to a state
slightly shifted away from the in focus state. Then, the direction
of astigmatism is found by applying FFT to a 2 dimensional image.
Then, while a relation between 2 astigmatism quantities of
different types is being sustained so that the astigmatism changes
only in this direction, the astigmatism quantities are changed to
search for a condition producing a clear image. For a more detailed
description of such techniques, reference may be had to a Japanese
Patent Prepublication No. Hei 10-106469.
In a still yet further technique, a point at which a change of a
Fourier transformation quantity is inverted is found by changing
the focal point by application of Fourier transformation of a 2
dimensional image. Then, a secondary electron image at a focal
point before an in focus state and a secondary electron image at a
focal point after the in focus state are measured. The direction of
astigmatism is then found from distributions of the maximums and
the minimums. Then, the astigmatism is corrected so that the
astigmatism changes in this direction. For a more detailed
description of such techniques, reference may be had to a in
Japanese Patent Prepublication No. Hei 9-82257.
In a still yet further technique, an astigmatism correction
quantity can be determined using sharpness magnitudes in 4
directions of scanning electron microscope images acquired by
increasing the position of the focal point are found. Then, the
position of the focal point is further increased till a maximum
value of each of the sharpness magnitudes is obtained. Finally, the
astigmatism correction quantity is found from the maximum values of
the sharpness magnitudes in these 4 directions. By using this
technique, an astigmatism correction quantity can be found from a
series of scanning electron microscope images with their focal
points shifted from each other. In this technique, however, maximum
values of sharpness magnitudes are used and whole changes in
sharpness magnitude are not taken into consideration. For a more
detailed description of such techniques, reference may be had to
U.S. Pat. No. 6,025,600.
Techniques that find a point at which the sharpness of a particle
image reaches a maximum by using a trial and error technique while
changing a total of 3 control quantities, namely, 2 astigmatism
correction quantities of different types and a focal point
correction quantity, can takes a relatively long time to complete
the correction process. If this occurs, a charged particle beam is
radiated to the sample for a long time, resulting in damage to the
sample due to electric charge and dirt accumulated in the sample.
In addition, when an automatic or visual adjustment is carried out
by using the sharpness as a measure, there is more likely a case in
which a condition can not be set to make astigmatism truly
disappear in dependence on a pattern on the sample.
Techniques that find a direction of astigmatism by vibrating the
focal point, and then perform a 1 dimensional scanning operation
repeatedly by vibrating the focal point while the astigmatism
correction quantity is being changed, may necessitate repeatedly
carrying out an operation to search for a condition in which focal
points in 2 directions coincide with each other, raising a problem
that it takes time to repeatedly carry out such an operation. In
addition, a 1 dimensional scanning operations performed on a sample
can leave a radiation mark on the sample. Furthermore, there is
also raised a problem of stability in the correction of the
astigmatism due to the fact that a signal with a sufficient
strength can not be obtained from some locations experiencing a 1
dimensional scanning operation because of non uniform texture on
the sample.
Techniques in which a 2 step adjustment is carried out in order to
vibrate an astigmatism correction quantity after a focal point is
vibrated raise a issue of a long time to complete the adjustment.
Also, an issue of severe damages inflicted on the sample can arise.
In addition, the processing to find the direction of astigmatism by
using the FFT can require a presumption of a uniform spectrum of an
image obtained with no astigmatism generated, imposing a limitation
on usable samples.
Many techniques do not determine a direction and a magnitude of
astigmatism from a particle image in a relatively stable manner,
nor compute a correction quantity from the direction and the
magnitude of astigmatism to be adopted in an astigmatism adjustment
device. Thus, the astigmatism correction quantity may need to be
changed, which can necessitate the correction quantity be examined
repeatedly on a trial and error basis. As a result, it takes time
to carry out the adjustment and, damage can be inflicted on the
sample due to contamination and buildup of a charge on the sample.
Furthermore, precision deterioration can occur for a location on
the sample with a coarse pattern scanned in a 1 dimensional
scanning operation.
Techniques in which the direction and the "strength" of astigmatism
are found by applying Fourier transformation to a 2 dimensional
image with a vibrating focal point, may not be able to find a
correction quantity for an astigmatism adjustment device from the
direction and the "strength" of astigmatism. There is thus raised a
problem of an inability to find a correction quantity for the
astigmatism adjustment means with a sufficiently high degree of
accuracy.
Techniques that determine an astigmatism correction quantity using
sharpness magnitudes in 4 directions can fail to find the
correction quantity in instances where the curve representing the
changes in sharpness magnitude is an asymmetrical or has a double
peaked character. In addition, an astigmatism correction quantity
may not always be found from a series of scanning electron
microscope images with their focal points shifted from each other
with a relatively high degree of accuracy. This is especially true
in cases were the sharpness curve tends to be asymmetric or have
double peaks, or when the astigmatism is large.
In order to solve the problems of prior art techniques, such as an
increased number of errors generated in correction of astigmatism
due to the use of maximum values of directional sharpness
magnitudes in an analysis of the directional sharpness magnitudes,
embodiments according to the present invention provide techniques
for finding a center of gravity of a curve representing the
directional sharpness magnitude. Techniques based on a center of
gravity can correct the center position of an area under a
asymmetrical or doubled peaked curve representing the directional
sharpness magnitude toward a region with a wider base in the case
of the asymmetrical curve or a region including the auxiliary hump
in the case of the double peaked curve. As a result, the
astigmatism can be corrected relatively surely as well as
relatively accurately.
Astigmatism correction quantities can include error due to an
effect of aberration other than astigmatism of the charged particle
optical system. Thus, if the magnitude of astigmatism is large, the
astigmatism is corrected repeatedly a plurality of times if
necessary till the change in astigmatism correction quantity is
reduced or converged to a small value. By this way, the failure in
correcting astigmatism can be prevented.
Specific embodiments of the present invention that implement a
charged particle beam apparatus, automatic astigmatism correction
techniques and samples for adjustments of astigmatism of a charged
particle beam are explained by referring to diagrams.
FIG. 1 illustrates an inspection and measurement apparatus in a
charged particle beam apparatus in a particular embodiment
according to the present invention. FIG. 1 illustrates a charged
particle optical system 10, a control system for controlling a
variety of elements comprising the charged particle optical system
10 and an image processing system for processing an image on the
basis of secondary particles or reflected particles detected by a
particle detector 16 in the charged particle optical system 10.
The charged particle optical system 10 comprises a charged particle
beam source 14 for emitting a charged particle beam such an
electron beam or an ion beam and an astigmatism corrector 60 for
correcting astigmatism of a charged particle beam emitted by the
charged particle beam source 14 by providing an electric field.
Further, a beam deflector 15 for deflecting the charged particle
beam emitted by the charged particle beam source 14 in a scanning
operation can also be part of charged particle optical system 10.
An objective lens 18 for converging the charged particle beam
deflected by the beam deflector 15 by means of a magnetic field; a
sample base 21 for mounting a sample 20 and fixing a calibration
target 62 beside the sample 20; an XY stage 46 for mounting and
moving the sample base 21; a grid electrode 19 set at an electric
potential close to that of the ground can also be included in
system 10. Further, a retarding electrode (not shown in the figure)
can be used for providing a negative electric potential relative to
the sample 20 and the calibration target 62 mounted on the sample
base 21 to an electron beam radiated by the charged particle beam
source 14 as a charged particle beam or a positive electric
potential relative to the sample 20 and the calibration target 62
to an ion beam radiated by the charged particle beam source 14 as a
charged particle beam. FIG. 1 also illustrates a height detection
sensor 13 for measuring the height of the sample 20 or another
object; and the particle detector 16 for detecting secondary
particles or reflected particles which emanate from the surface of
the sample 20 as a result of radiation of a charged particle beam
to the sample 20 and are reflected typically by a reflective
plate.
It should be noted that the astigmatism corrector 60 can be
implemented by an astigmatism correction coil based on a magnetic
field or an astigmatism correction electrode based on an electric
field. The objective lens 18 can be implemented by an objective
coil based on a magnetic field or a static objective lens based on
an electric field. In addition, the objective lens 18 can also be
provided with a coil 18a for correction of the focal point. An
astigmatism adjustment means comprises the astigmatism corrector 60
and an astigmatism correction circuit 61.
A stage control unit 50 drives and controls the movement or the
travel motion of the XY stage 46 while monitoring the position or
the displacement of the XY stage 46 on the basis of a control
command issued by a whole control unit 26. It should be noted that
the XY stage 46 is provided with a position monitoring length
measurement unit for monitoring the position or the displacement of
the XY stage 46. The monitored position or displacement of the XY
stage 46 can be supplied to the whole control unit 26 through the
stage control unit 50.
A focal position control unit 22 drives and controls the objective
lens 18 in order to adjust the focal point of the charged particle
beam to a position on the sample 20 on the basis of height
information of the surface of the sample 20 measured by a height
detection sensor 13 in accordance with a command issued by the
whole control unit 26. It should be noted that, by adding a Z stage
to the XY stage 46, the focal point can be adjusted by driving and
controlling the Z stage in place of the objective lens 18. The
objective lens 18 or the Z stage, the focal position control unit
22 and other components constitute a focus point control means.
A deflection control unit 47 supplies a deflection signal to the
beam deflector 15 in accordance with a control command issued by
the whole control unit 26. At that time, a correction is added to
the deflection signal so as to compensate for a variation in image
magnification accompanying a change in surface height of the sample
20 and for an image rotation accompanying control of the objective
lens 18.
A grid potential adjustment unit 48 adjusts the close to the ground
electric potential applied to the grid electrode 19 provided at a
position above and close to the sample 20 in accordance with a
potential adjustment command issued by the whole control unit 26. A
sample base potential adjustment unit 49 adjusts an electric
potential applied to the retarding electrode provided above the
sample base 21 in accordance with a potential adjustment command
issued by the whole control unit 26. A negative or positive
electric potential applied to the sample 20 by the grid electrode
19 and the retarding electrode reduces the velocity of an electron
beam or an ion beam traveling between the objective lens 18 and the
sample 20 in order to raise the resolution in a low acceleration
voltage area.
A beam source potential adjustment unit 51 adjusts an electric
potential applied to the charged particle beam source 14 in
accordance with a command issued by the whole control unit 26 to
regulate a beam current or an acceleration voltage of a charged
particle beam emitted by the charged particle beam source 14.
The beam source potential adjustment unit 51, the grid potential
adjustment unit 48 and the sample base potential adjustment unit 49
are controlled by the whole control unit 26 so that a particle
image with a desired quality can be detected by the particle
detector 16.
In processing to correct astigmatism and a focal point, an
astigmatism adjustment unit 64 provided by the present invention
issues a control command to change the focal point (also referred
to as a focus f) to the focal position control unit 22. Receiving
this command, the focal position control unit 22 drives and
controls the objective lens 18 so as to change the focus f of a
charged particle beam radiated to an area on the sample 20 or the
calibration target 62 with a certain pattern created therein. The
pattern typically includes edge components of about the same
quantity in directions shown in FIGS. 4(a) and (b). By doing so,
the particle detector 16 detects a plurality of particle image
signals with different loci f. Each of the particle image signals
is converted by an A/D converter 24 into a particle digital image
signal or digital image data to be stored in an image memory 52,
being associated with a respective focus command value f output by
the astigmatism adjustment unit 64. Then, an astigmatism and focus
correction quantity computation image processing circuit 53 reads
out the particle digital image signals with different foci f from
the image memory 52, finding directional sharpness magnitudes d0
(f), d45 (f), d90 (f) and d135 (f) for each of the particle digital
image signals. The astigmatism and focus correction quantity
computation image processing circuit 53 then finds focus values f0,
f45, f90 and f135 that generate peak values of the directional
sharpness magnitudes d0 (f), d45 (f), d90 (f) and d135 (f)
respectively. Then, the astigmatism and focus correction quantity
computation image processing circuit 53 finds the astigmatism (to
be more specific, an astigmatism vector (dx and dy) or an
astigmatism direction .alpha. and an astigmatism magnitude .delta.)
and a focal offset value z from the focus values f0, f45, f90 and
f45, supplying the astigmatism and the focal offset value z to the
whole control unit 26 to be stored in a storage unit 57. From a
relation between the astigmatism and the astigmatism correction
quantity found in advance, the whole control unit 26 computes
astigmatism correction quantities .DELTA.stx and .DELTA.sty for the
astigmatism which is found as described above and stored in the
storage unit 57. It should be noted that the relation between the
astigmatism and the astigmatism correction quantity represents a
characteristic of the astigmatism corrector 60. By the same token,
from a relation representing a characteristic of the objective lens
18 found in advance, the whole control unit 26 computes a focal
point correction quantity for the focal offset value z found as
described above and stored in the storage unit 57. The astigmatism
correction quantities .DELTA.stx and .DELTA.sty and the focal point
correction quantity found in this way are supplied to the
astigmatism adjustment unit 64.
The astigmatism adjustment unit 64 passes on the astigmatism
correction quantities .DELTA.stx and .DELTA.sty received from the
whole control unit 26 to the astigmatism adjustment circuit 61 so
as to allow the astigmatism corrector 60 to correct the astigmatism
of the charged particle beam. The astigmatism corrector 60 is an
astigmatism correction coil based on a magnetic field or an
astigmatism correction electrode based on an electric field. By the
same token, the astigmatism adjustment unit 64 passes on the focal
point correction quantity to the focal position control unit 22 to
control a coil current flowing to the objective lens 18 or a coil
current flowing to a focal point correction coil 18a. As a result,
the focal point is corrected.
As another technique, the XY stage 46 is provided with a Z stage as
described above. In this case, the astigmatism adjustment unit 64
outputs a control command to vibrate the focal point or to change
the focus to the stage control unit 50 through the whole control
unit 26 or directly. Receiving this command, the stage control unit
50 vibrates the focal point by driving a Z shaft of the XY stage
46. Thus, a particle image with a vibrating focal point is obtained
from the particle detector 16. In the astigmatism and focus
correction quantity computation image processing circuit 53,
astigmatism and focal point correction quantities are found and the
computed focal point correction quantity is fed back to the Z shaft
of the XY stage 46 to allow a correction to be carried out. Of
course, a component for acquiring an image with a vibrating focal
point and a component subjected to the final focal point correction
can be provided separately. To put it concretely, one of them is
implemented by the focal position control unit 22 while the other
is implemented by the Z shaft of the XY stage 46. As another
alternative, the two components are combined to execute control by
using both at the same time. In either case, the focal position
relative to the location of the sample 20 or the calibration target
62 needs to be controlled to a desired distance. It should be noted
that the technique of controlling the objective lens 18 offers a
response characteristic superior to that of the technique of
controlling the Z shaft in at least one specific embodiment.
As described above, the correction of the astigmatism and the focal
point is based on control executed by the astigmatism adjustment
unit 64 in accordance with a command issued by the whole control
unit 26. As a result, the whole control unit 26 receives a particle
image with the astigmatism and the focal point thereof corrected
from the image memory 52 directly or through the astigmatism and
focus correction quantity computation image processing circuit 53
and displays the particle image on a display means 58 to allow the
user to visually form a judgment as to whether the correction of
the astigmatism and the like is correct or incorrect.
Furthermore, in an inspection or a measurement, the XY stage 46 is
controlled to take a predetermined position on the sample 20 to the
visual field of the charged particle optical system. Then, a
particle image signal obtained by the particle detector 16 is
converted by the A/D converter 24 into a particle digital image
which is then stored in an image memory 55. Subsequently, an
inspection and measurement image processing circuit 56 measures
dimensions of a fine pattern created on the sample 20 or inspects
the sample 20 for a defect of a fine pattern or a defect such as an
infinitesimal foreign material on the basis of a detected particle
digital image signal stored in the image memory 55. Results of the
measurement and the inspection are supplied to the whole control
unit 26. At that time, by correcting the astigmatism and the focal
point in accordance with the technique provided by the present
invention at least periodically, it is possible to implement a
measurement and an inspection based on a particle image with the
astigmatism always corrected.
It should be noted that, in an inspection of a defect or the like
based on a particle image, the inspection and measurement image
processing circuit 56 may generate a reference particle digital
image signal to be used as a comparison object by delaying a
detected detection particle digital image signal and then comparing
a current detection particle digital image signal with the
reference particle digital image at a position corresponding to
that of the current detection particle digital image signal to
detect a mismatch or a difference between the 2 signals as a defect
candidate. Then, the inspection and measurement image processing
circuit 56 carries out processing to recognize a characteristic
quantity of each defect candidate and form a judgment as to whether
or not to eliminate false information from characteristic
quantities in order to inspect the sample 20 for a real defect.
Slightly affected by things such as electrical charge and dirt
accumulated on the sample 20 and damage inflicted on the sample 20,
the optical height detection sensor 13 is capable of detecting
variations in height of the sample 20 at positions being measured
or inspected. The variations are fed back to the focal position
control unit 22 so that an in focus state can be sustained all the
time. When the optical height detection sensor 13 is used in this
way, the astigmatism and the focal position can be automatically
adjusted at another position on the sample 20 or at the calibration
target 62 provided on the sample base 21 in advance or periodically
in the course of a measurement or an inspection. Thus, radiation of
a converged charged particle beam for an automatic adjustment of
the astigmatism and the focal point can be omitted or reduced
considerably, allowing effects of electric charge and dirt
accumulated on the sample 20 and a damage inflicted on the sample
20 to be eliminated.
The following description explains automatic adjustment of the
astigmatism and the focal point in a converged charged particle
optical system provided by a specific embodiment according to the
present invention. In this specific embodiment according to the
present invention, the amount of astigmatism and the focal offset
are found from a small number of 2 dimensional particle images. The
amount of astigmatism and the focal offset are then converted into
correction quantities of the astigmatism and the focal offset
respectively at the same time in a one time correction process.
FIG. 2 is a diagram showing a configuration of 2 coil sets each
comprising a plurality of coils. Based on magnetic fields, these 2
coil sets serve as an embodiment of the astigmatism corrector 60.
In the configuration of the 2 coil sets each comprising a plurality
of astigmatism correction coils, a current flowing through one of
the coil sets has an effect of expanding a beam in a certain
direction while shrinking the beam in a direction perpendicular to
the certain direction. If the 2 coil sets are controlled
respectively by a combination of 2 magnetic fields stx and sty
shifted from each other by a phase difference of 45 degrees as
shown in FIG. 2, the astigmatism can be adjusted in any arbitrary
direction by a necessary amount. Of course, the astigmatism
corrector 60 can also be designed to comprise electrodes based on
magnetic fields.
Next, states of astigmatism are explained by referring to FIG. 3. A
column on the left side represents states of converged charged
particle beams with astigmatism thereof corrected. A state on the
top of a column is a case with a high focal position (Z>0). A
state in the middle of a column represents an in focus state (Z=0).
A state at the bottom of a column is a case with a low focal
position (Z<0). As indicated by the column on the left side, at
the in focus position, the charged particle beam is converged on a
small point. At positions above and below the in focus position,
the diameter of the beam increases symmetrically with respect to
the in focus position. The column in the middle of FIG. 3
represents states obtained as a result of flowing an stx current.
For Z>0, the beam is expanded in the horizontal direction. For
Z<0, on the other hand, the beam is expanded in the vertical
direction. At the in focus position, the beam forms a true circle
but the diameter of the circle is not reduced to a sufficiently
small value. The column on the right side of FIG. 3 represents
states obtained as a result of flowing an sty current. At the
positions shifted from the in focus position, the orientation of
elliptical beam is rotated by 45 degrees in directions opposite to
each other so that the direction of the major axis of the ellipse
for Z>0 is perpendicular to that for Z<0. By combining an stx
current with an sty current, astigmatism of any arbitrary direction
can be generated in any arbitrary direction so as to cancel
pre-adjustment astigmatism of the charged particle optical system.
As a result, the astigmatism can be corrected.
As shown in FIG. 3, in a state with generated astigmatism, at a
position shifted from the in focus position, the charged particle
beam blurs into an elliptical shape. To be more specific, at +Z and
-Z positions sandwiching the focal point, the elliptical shapes of
the beam are thinnest and the major axes of the ellipses are
oriented in directions perpendicular to each other. The magnitude
of the astigmatism is represented by the focal distance 2Z between
the 2 shifted points and the direction of the astigmatism is
represented by the direction of the ellipse. The focal distance 2Z
between the 2 shifted points is referred to astigmatism which is
denoted by notation .delta. shown in FIG. 6. On the other hand, the
direction of the astigmatism is represented by a main point main
axis direction .alpha. shown in FIG. 6. An astigmatism vector can
be denoted by notation (dx, dy).
Next, correction of the astigmatism and the focal point is
explained by referring to FIGS. 4 to 7. FIGS. 4(a) and (b) are
diagrams each showing an embodiment of a pattern created on the
sample 20 or the calibration target 62 and used for correction of
the astigmatism and the focal point. Any pattern can be used as a
pattern for correction of the astigmatism and the focal point as
long as the pattern includes edge components of about the same
quantity in at least 3 directions in which astigmatism is
generated. FIG. 4 (a) shows an embodiment with 4 patterns of
straight lines created in different areas. The direction of the
straight lines varies from pattern to pattern. On the other hand,
FIG. 4(b) shows curve shaped patterns which have edge components in
4 directions and are laid out 2 dimensionally at a uniform pitch.
In the case of the sample 20, in particular, a pattern created
thereon to include edge components of about the same quantity in at
least 3 directions is usable. In this case, however, it is
necessary to supply information on a position to create this
pattern to the whole control unit 26 in advance by using an input
means 59 and store the information typically in the storage unit
57, or it is necessary for the operator to specify a position on
the sample 20 for each correction of the astigmatism and the focal
position. In addition, it is a matter of course that information on
a position to install the calibration target 62 on the sample base
21 is supplied to the whole control unit 26 in advance by using the
input means 59 and stored in the storage unit 57 in advance.
The whole control unit 26 supplies the information on a position of
a pattern for correction of the astigmatism and the focal point to
the stage controller 50. In accordance with the information, the
stage controller 50 drives and controls the XY stage 46 to move the
pattern for correction of the astigmatism and the focal point to a
position in close proximity to an optical axis of the charged
particle optical system.
(1) At a step S51 of a flowchart shown in FIG. 5, while the charged
particle beam is being radiated to the pattern for correction of
the astigmatism and the focal point in a scanning operation in
accordance with a command issued by the whole control unit 26 to
the deflection control unit 47 and the focus f is being changed in
accordance with a command issued by the astigmatism adjustment unit
64 to the focal position control unit 22, the particle detector 16
acquires a plurality of images and stores them in the image memory
52. The astigmatism and focus correction quantity computation image
processing circuit 53 computes directional sharpness magnitudes (at
0 degrees, 45 degrees, 90 degrees and 135 degrees) as d0 (f), d45
(f), d90 (f) and d135 (f) respectively as shown in FIG. 6(a). It
should be noted that it is possible to acquire a focus f as a value
specified in a command issued by the astigmatism adjustment unit 64
to the focal position control unit 22.
(2) At a next step S52, the astigmatism and focus correction
quantity computation image processing circuit 53 finds center
positions p0, p45, p90 and p135 shown in FIG. 6(a) for each curve
representing the 4 directional sharpness magnitudes as a function
of focus f in one of the directions.
(3) At a next step S53, the astigmatism and focus correction
quantity computation image processing circuit 53 finds the
direction .alpha. and the magnitude .delta. of a focal shift (or
astigmatism) caused by directional astigmatism from the center
positions p0, p45, p90 and p135 on the basis of a sinusoidal
function shown in FIG. 6(b). The astigmatism and focus correction
quantity computation image processing circuit 53 also finds the
focal offset z. The astigmatism direction .alpha., the astigmatism
magnitude .delta. and the focal offset z are supplied to the whole
control unit 26 to be stored in the storage unit 57. It should be
noted that, at the step S53, the astigmatism vector (dx, dy) can
also be found in place of the direction .alpha.and the magnitude
.delta. of the astigmatism. The magnitude .delta. of the
astigmatism can be expressed by Eq. (1) given below. On the other
hand, the direction .alpha. of the astigmatism can be expressed by
Eq. (2) given below. As for the focal offset z, Eq. (3) given below
is applicable. ##EQU1##
It should be noted that the storage unit 54 is used for storing
software including a program for finding the directional sharpness
magnitudes d0 (f), d45 (f), d90 (f) and d135 (f) described above, a
program for finding their center positions p0, p45, p90 and p135
from the directional sharpness magnitudes d0 (f), d45 (f), d90 (f)
and d135 (f) respectively and a program for finding the astigmatism
and the focal offset. The astigmatism and focus correction quantity
computation image processing circuit 53 has a configuration capable
of carrying out processing based on these programs. Of course, the
storage unit 54 can be implemented as a ROM or the like.
(4) At a next step S54, the whole control unit 26 becomes capable
of converting or apportioning astigmatism (.alpha. and .delta. or
(dx, dy)) into or to required astigmatism correction quantities (1,
2) (.DELTA.stx, .DELTA.sty) by using a relation between variations
in astigmatism control values (stx, sty) and variation quantities
(or sensitivities) in astigmatism direction .alpha. and astigmatism
magnitude .delta. or in astigmatism vector (dx, dy). This relation
is a characteristic of the astigmatism corrector 60 which is found
in advance. At the next step S55, the whole control unit 26 becomes
capable of setting the astigmatism correction quantities (1, 2)
(.DELTA.stx, .DELTA.sty) and the focal offset value z to be
supplied to the astigmatism adjustment unit 64. It should be noted
that the astigmatism correction quantities (1, 2) (.DELTA.stx,
.DELTA.sty) and the focal value z may be computed by the
astigmatism and focus correction quantity computation image
processing circuit 53, which receives the characteristics of the
astigmatism corrector 60 and the objective lens 18 from the whole
control unit 26 instead of being computed directly by the whole
control unit 26.
(5) The astigmatism adjustment unit 64 supplies the focal offset
value z received from the whole control unit 26 to the focal
position control unit 22, which corrects an objective lens current
flowing through the objective lens 18 or a focus correction coil
current flowing through the focal point correction coil 18a. The
astigmatism adjustment unit 64 also supplies the astigmatism
correction quantities (.DELTA.stx, .DELTA.sty) received from the
whole control unit 26 to an astigmatism correction circuit 61,
which corrects an astigmatism correction coil current or an
astigmatism correction static voltage. In this way, the correction
of the astigmatism and the adjustment of the focal point can be
carried out in a one time operation.
(6) In the case of a small amount of astigmatism, auto stigma
processing is completed in the one time operation described above.
In the case of a large amount of astigmatism, on the other hand,
the processing can not be completed in a one time operation due to
causes of aberration other than the astigmatism. Examples of
aberration other than the astigmatism are high order astigmatism
and image distortion. In this case, the flow of the procedure goes
back to (1) to carry out the auto stigma processing once again. The
loop is executed repeatedly till the focal offset z and the
astigmatism correction quantities (.DELTA.stx, .DELTA.sty) are
reduced to small values.
By adopting the technique described above, the astigmatism and the
focal point can be adjusted at a relatively high speed in a one
time operation with only few small damages inflicted on the sample
20 and the calibration target 62. In addition, while the focal
point is being changed, the directional sharpness magnitudes of
images of the same sample 20 or the same calibration target 62 are
compared with each other to find astigmatism. Thus, the astigmatism
and the focal point can be adjusted with a relatively high degree
of precision in a one time operation without relying on a pattern
on the sample 20 or the calibration target 62, that is, a pattern
for correction of the astigmatism and the focal point. The only
requirement for a pattern created on the sample 20 or the
calibration target 62 is that the pattern must include edge
components of about the same quantity in each direction.
As described above, directional astigmatism magnitudes of 4
different types, that is, astigmatism at .theta.=0 degrees, 45
degrees, 90 degrees and 135 degrees, are used. It should be noted,
however, that the angle .alpha. does not have to be the 4
directions provided that the astigmatism direction .alpha. and the
astigmatism magnitude .delta. are known. Directional astigmatism
magnitudes d.theta.(f) at any arbitrary number of angles .theta. in
at least 3 directions can be used. For each angle .theta., the
center position p.theta. of the curve d.theta.(f) is found. Then,
the amplitude and a phase of a sinusoidal wave for the center
position p.theta. are found as an astigmatism magnitude .delta. and
an astigmatism phase .alpha. respectively. It should be noted that
the waveform does not have to be sinusoidal. Instead, an almost
sinusoidal waveform can be used.
The following description explains embodiments each used for
finding directional sharpness magnitudes of a particle image in the
astigmatism and focus correction quantity computation image
processing circuit 53.
As a first embodiment, the particle detector 16 is used for
detecting and observing a particle image by radiating a charged
particle beam to a sample (target) 62 for automatic astigmatism
correction based on a pattern with a direction varying from area to
area as shown in FIG. 7(a) in a scanning operation. The directional
sharpness magnitude d.theta. is found by measuring the amplitude of
a particle image in each of the areas. The amplitude can be found
by calculating the difference between the maximum value of s (x, y)
and the minimum value of s (x, y) for each of the areas, or by
calculating the differential (V=.SIGMA..sub.xy (s (x,
y)-smean).sup.2 /N) of the concentration value (or the gradation
value) s (x, y) of the particle image for each of the areas. As an
alternative, the sum .SIGMA..sub.xy (t (x, y).sup.2.vertline.t (x,
y).vertline. where .vertline.t (x, y).vertline. is the absolute
value of a differential t (x, y) of a 2 dimensional differential
result s (x, y) of typically Laplacian and the sum of squares
.SIGMA..sub.xy (t (x, y).sup.2 are found. The sum found in this way
is defined as a directional sharpness magnitude d.theta.. The
angular direction .theta. can be defined in any arbitrary way. In
the case of the example shown in the figure, the angular direction
.theta. is defined in a clockwise direction starting at 0 degrees
for the orientation of the direction of the normal line of the
pattern coinciding with the horizontal direction. Directions of the
pattern are not restricted to the 4 directions shown in the figure.
Instead, the angle range of 180 degrees can be divided into n
almost equal segments and pattern directions can then be determined
by possible combinations of any of the n segments where n is an
integer equal to or greater than 3.
A second embodiment implements a sample 20 or a calibration target
62 with a pattern like one shown in FIG. 7(b), and finds the
directional sharpness magnitude d.theta. by calculating directional
differentials for a particle image detected by the particle
detector 16. A directional differentiation is implemented by
carrying out convolution processing for an image on a mask like one
shown in the figure. Then, a sum of squares of values at points for
an image obtained as a result of the differentiation is computed as
a directional sharpness magnitude d.theta.. It should be noted that
the differentiation mask shown in the figure is typical. That is,
the use of this mask is not mandatory. Any mask can be used as long
as a requirement for a mask used for computing directional
differentials is satisfied. The requirement is that 2 values at any
2 positions symmetrical with respect to a certain axis have signs
opposite to each other but about equal magnitudes. There are a
variety of achievable differentiation mask variations for
suppressing noise and improving selectability of the
differentiation direction. In addition, it is also necessary to
select a filtering means prior to computation of image
differentials and a means for shrinking the image. The selected
means are means suitable for the image. Furthermore, by carrying
out a directional differentiation after rotation of the image, a
simple 0 degree differentiation or a 90 degree differentiation can
also be adopted to perform a directional differentiation at any
arbitrary directional angle .theta..
The following description explains an embodiment for finding the
center position p.theta. for a directional sharpness magnitude
expressed as a function d.theta.(f) of focal point f in the
astigmatism and focus correction quantity computation image
processing circuit 53. As a technique for finding the center
position p.theta., a proper method can be selected. The selected
method can be a technique for finding the center position p.theta.
as a center position of a function such as a second order function
or a Gaussian function applied to values preceding and succeeding
the position of the focal point f giving a maximum value of the
function d.theta.(f). Another selected method can be a technique
for finding the center position p.theta. as a center of gravity for
points at which the values of the function d.theta.(f) are than
greater than a threshold value.
The following description explains an embodiment for finding the
astigmatism correction quantity in the whole control unit 26 from
information on the astigmatism received from the astigmatism and
focus correction quantity computation image processing circuit 53.
When the 4 directions p0, p45, p90 and p135 at angles 0 degrees, 45
degrees, 90 degrees and 135 degrees respectively are used, the
astigmatism and focus correction quantity computation image
processing circuit 53 computes the astigmatism vector (dx,
dy)=(p0-p90, p45-p135) and supplies the astigmatism vector to the
whole control unit 26. Next, the whole control unit 26 apportions
the astigmatism vector to the astigmatism correction quantities
.DELTA.stx and .DELTA.sty in accordance with Eq. (4) as
follows:
where mxx, mxy, myx and myy are astigmatism correction quantity
apportioning parameters computed in advance on the basis of a
characteristic of the astigmatism corrector 60 and stored typically
in the storage unit 57. The astigmatism adjustment unit 64 passes
on the astigmatism correction quantities .DELTA.stx and .DELTA.sty
received from the whole control unit 26 to the astigmatism
adjustment circuit 61, requesting the astigmatism adjustment
circuit 61 that the astigmatism correction quantities .DELTA.stx
and .DELTA.sty be changed by .beta..DELTA.stx and .beta..DELTA.sty
where notation .beta. denotes a correction quantity reduction
coefficient. In response to such a request, the astigmatism
adjustment circuit 61 requests the astigmatism corrector 60 to
change the astigmatism correction quantities .DELTA.stx and
.DELTA.sty by .beta. .DELTA.stx and .beta..DELTA.sty.
In the whole control unit 26, a focal point correction quantity is
set at (p0+p45+p90+p135)/4 due to the fact that the focal offset z
received from the astigmatism and focus correction quantity
computation image processing circuit 53 is an average value of
focal positions in the respective directions. Thus, the astigmatism
adjustment unit 64 passes on the focal point correction quantity
received from the whole control unit 26 typically to the focal
position control unit 22. The focal position control unit 22 then
corrects the objective lens 18 by the focal point correction
quantity.
It should be noted that another embodiment is achievable. In this
alternative embodiment, the astigmatism and focus correction
quantity computation image processing circuit 53 finds the
astigmatism magnitude .delta.=.vertline.(dx, dy).vertline. and the
astigmatism direction .alpha.=1/2arctan (dy/dx) from the
astigmatism vector (dx, dy) and supplies the astigmatism magnitude
.delta. and the astigmatism direction .alpha. to the whole control
unit 26. The whole control unit 26 then converts the astigmatism
magnitude .delta. and the astigmatism direction .alpha. into
astigmatism correction quantities .DELTA.stx and .DELTA.sty.
When directional sharpness magnitudes p.theta. in n directions are
used where n is an integer equal to or greater than 3, the
astigmatism and focus correction quantity computation image
processing circuit 53 applies a sinusoidal waveform to these pieces
of data to find the astigmatism magnitude .delta., the astigmatism
direction .alpha. and the focal offset z from a phase, an amplitude
and an offset thereof.
If the astigmatism correction quantities are changed, the focal
point may be affected by the changes, hence, being shifted
slightly. In this case, typically, the whole control unit 26
multiplies the astigmatism correction quantities .DELTA.stx and
.DELTA.sty by their respective proper coefficients and uses the
products obtained as changes in astigmatism correction quantities
.DELTA.stx and .DELTA.sty.
The following description explains another embodiment of the
present invention for correcting the astigmatism and the focal
point at an even higher speed by referring to FIGS. 8 and 9. In
this embodiment, the surface of the calibration target 62 is
inclined as shown in FIG. 8(a) or has a staircase like shape as
shown in FIG. 8(b). Reference numerals 62a and 62b denote the
former and latter calibration targets respectively which each have
a proper pattern created on the surface thereof. The calibration
target 62a or 62b is placed on the sample base 21 as shown in FIGS.
1 and 10. Thus, a particle image of the calibration target 62a or
62b which also serves as a sample has a focal point f varying from
area to area. It should be noted that the difference in height
between a reference point on the calibration target 62a or 62b and
the surface of the sample 20 is measured in advance. A technique
for automatically correcting a height for both the calibration
target 62 and the sample 20, and a technique for measurement using
an optical height sensor to be described later.
By using the calibration target 62a or 62b shown in FIG. 8(a) or 8
(b) respectively, it is possible to obtain a particle image that
has a focus f varying from area to area. Thus, a flowchart shown in
FIG. 9 is different from the flowchart shown in FIG. 5 in that a
particle image obtained at a step S51' of the former has a height
(or a focus f) varying from area to area and includes edge
components of about equal quantities in at least 3 directions. At
the same step, directional sharpness magnitudes p.theta.(f) of such
a particle image in the directions are computed. Thereafter, the
quantities for correcting the astigmatism and the focal point are
found to be used in an adjustment of the astigmatism and the focal
point in the same way as the steps S52 to S55 of the flowchart
shown in FIG. 5. In this way, by acquiring only 1 particle image,
the astigmatism and the focal point can be corrected at a
relatively high speed.
In addition, the same effect as that of the embodiment described
above can be obtained by using a flat surface calibration target 62
or a flat surface sample 20. In the case of such a calibration
target 62 or such a sample 20, a particle image is taken while the
focal point is being changed at a relatively high speed. In this
way, it is possible to obtain an image with a focal point varying
from area to area as is the case with the embodiment described
above. As a result, by acquiring only 1 particle image, the
astigmatism and the focal point can be corrected at a relatively
high speed.
The following description explains a relation between inspections
or measurements of a substrate and astigmatism and focal point
corrections. The actual sample 20 serving as a substrate subjected
to an inspection or a measurement is mounted on the sample base 21.
Then, at least information on a predetermined position on the
object substrate 20 is supplied to the whole control unit 26 by
using an input means 59 to be stored in the whole control unit 26.
The predetermined position reported to the whole control unit 26 is
a position subjected to an inspection or a measurement. The input
means 59 is typically a recording medium or a network. Thus, when
the inspection or the measurement of the object substrate 20 is
implemented, the XY stage 46 is controlled by a command issued by
the whole control unit 26 to take the predetermined position on the
object substrate 20 into the visual field of the charged particle
optical system. Subsequently, a charged particle beam is radiated
onto the surface of the object substrate 20 to carry out a scanning
operation and a particle image obtained as a result of the
radiation is detected by the particle detector 16. The particle
image is then subjected to A/D conversion before being stored in
the image memory 55. Then, the inspection and measurement image
processing circuit 56 processes the particle image in order to
carry out the inspection or the measurement. At that time, at the
position subjected to the inspection or the measurement, the
astigmatism and the focal position are corrected by using the
method provided by the present invention. As a result, it is
possible to carry out an inspection or a measurement based on a
particle image with the astigmatism thereof always corrected.
In the case of an inspection and measurement apparatus provided
with typically an optical height detection sensor 13 wherein the
object substrate is affected by electric charge, dirt, a damage or
the like only slightly, a converged charged particle beam is
radiated to the sample 20 in a scanning operation for an inspection
or a measurement instead of using a height detected by the optical
height detection sensor 13 at each position on the sample 20
subjected to the inspection or the measurement as feedback
information in the processing of the focal point and radiating the
converged charged particle beam in a scanning operation for an
adjustment of the focal point and the astigmatism. Thus, the effect
of electric charge, dirt, a damage or the like only on the object
substrate 20 or the sample 20 can be suppressed to a minimum. In
this case, the astigmatism and the focal point are automatically
adjusted by using a separate position on the sample 20 or the
calibration target 62 provided on the sample base 21. The automatic
adjustment is carried out in advance or periodically in the course
of the inspection or the measurement. Further, the calibration
target 62 can be a sample having an inclined or staircase like
surface shown in FIG. 8 or a sample having a flat surface as shown
in FIG. 1.
In the automatic adjustment of the astigmatism and the focal point
according to the present invention as described above, shifts of
the focal point and the astigmatism which occur with the lapse of
time are corrected. In the automatic adjustment of the astigmatism
and the focal point according to the present invention, however, it
is necessary to adjust a detected offset with the optical height
detection sensor 13 in advance. Differences in height or variances
in height among positions on the actual sample 20 or the object
substrate 20 are detected by the optical height detection sensor 13
in the correction of the focal point. Thus, only during an
inspection or a measurement is an astigmatism free converged
charged particle beam radiated to the actual sample 20 in an in
focus state in a scanning operation. Therefore, a particle image
can be detected in a state where the effect of electric charge,
dirt, a damage or the like only on the actual sample 20 can be
suppressed to a minimum. As a result, the object substrate 20 can
be inspected or measured with a relatively high degree of
precision.
In addition, when it is desired to calibrate not only an offset
between the optical height detection sensor 13 and the focal
position control unit 22, but also the gain, a plurality of
calibration targets 62 with their heights known are provided in
advance. Then, an automatic correction of the focal point and a
detection by using the optical height detection sensor 13 are both
carried out on each of the calibration targets 62 in order to
calibrate the gain and even linearity. Furthermore, an automatic
correction of the focal point and a detection by using the optical
height detection sensor 13 can both be carried out on each of the
calibration targets 61 or the samples 20 while the height of the
calibration target 61 or the sample 20 is being varied by using the
Z shaft of the XY stage 46 in order to calibrate the gain and even
the linearity.
Moreover, while the XY stage 46 is being moved continuously in the
horizontal direction as shown in FIG. 10, the beam deflector 15 is
driven to radiate a converged charged particle beam in a scanning
operation in a direction crossing the moving direction of the XY
stage 46 almost perpendicularly in particular to allow the particle
detector 16 to continuously detect a particle image. In an
inspection or a measurement carried out at a relatively high speed,
the control described below is executed. A height detected by the
optical height detection sensor 13 is fed back to the focal
position control unit 22 and the deflection control unit 47 all the
time. By doing so, while a shift of the focal point and a rotation
of the deflection are being corrected all the time, a particle
image is detected. It is thus possible to implement a relatively
high speed inspection or a relatively high speed measurement with a
relatively high degree of precision over the entire surface of the
actual sample 20. It should be noted that, instead of driving the
focal position control unit 22 to correct the focal point, the Z
shaft of the XY stage 46 can of course be driven to result in the
same effects. In the mean time, the operation is shifted to the
calibration target 62 periodically as shown in FIG. 10 to
automatically correct the focal point and the astigmatism. It is
thus possible to carry out a relatively high precision and high
sensitivity inspection or a relatively high precision and high
sensitivity measurement based on a particle image with the focal
point and the astigmatism thereof corrected with a relatively high
degree of precision.
The following description explains a method of finding the center
position of an area under a curve representing the directional
sharpness magnitude wherein a function having a peak such as a
quadratic function or a Gaussian function is used to represent the
curve with reference to FIG. 11. As shown in the figure, a point at
which a maximum value of the sharpness magnitude is located is
found. Then, a convex function such as a quadratic function or a
Gaussian function is applied to N data points preceding and
succeeding the maximum value point. For N=3, it is possible to find
such parameters that all the data points are placed on the curve of
the quadratic function or the Gaussian function. Thus, the center
position of an area under the curve representing the directional
sharpness magnitude can be found by interpolation.
By a maximum position or interpolation based on a maximum value
position, however, an error will be generated particularly in the
case of a large magnitude of astigmatism. This problem is shown in
FIG. 12. Consider a sharpness magnitude in the 0 degree direction
in a case in which astigmatism is generated in the approximately
.+-.45 degree direction. Thus, if the spot of the charged particle
beam is in an in focus state in the .+-.45 degree direction, the
spot cross section length in the 0 degree direction is narrowed. In
an in focus state, on the other hand, the spot cross section length
in the 0 degree direction is widened. Generally, the narrower the
spot cross section length in the 0 degree direction, the greater
the sharpness magnitude. Thus, a sharpness quantity curve in a
direction with no generated astigmatism tends to exhibit a double
peaked characteristic for a large sharpness magnitude as is the
case with d0 (f) and d90 (f) curves shown in FIG. 12(b). If an
interpolation based only on a maximum value position is adopted in
such a case, a one sided position like a point B shown in FIG.
12(c) will be determined to be the center position of the area
under the curve representing the directional sharpness magnitude.
In this case, a value close to the maximum p45 of the function d45
(f) is taken as the center of the area under the curve representing
the function d0 (f). In the example of FIG. 12, when maximum
position is used, p0 becomes very close to p45 and p90 becomes very
close to p135. In this case, the estimated .+-.45 degree component
of the astigmatic focus distance becomes twice as large as it
should be. If this astigmatic focus distance is used for the
astigmatic correction, the astigmatism in this direction is
corrected too much, resulting in unstable behavior. In contrast,
depending on the technique of searching for a maximum value, a
point C representing the maximum value may be taken as the center
of the area under the curve representing the function d0 (f). In
this case, almost no correction occurs for astigmatism in the
.+-.45 degree direction. In order to find the astigmatism magnitude
and the astigmatism direction correctly as explained earlier by
referring to FIG. 6, however, it is necessary to determine a middle
point between the points B and C such as a point A shown in FIG.
12(c) as the center of the area under the curve representing the
function d0(f).
As described above, in select embodiments according to the present
invention, a point between the points B and C is found as the
center of the area under the curve representing the astigmatism
magnitude in dependence on the sizes of the peaks of the points B
and C. There are a variety of techniques of determining such a
middle point. Some embodiments implementing these techniques are
explained as follows. It should be noted that the scope of the
present invention is not limited to the described embodiments. That
is, the scope of the present invention includes the use of any
technique to find a middle value in accordance with the sizes of
the peaks.
FIG. 13 is an explanatory diagram showing a technique based on the
center of gravity. According to this technique, a maximum value is
found. Then, the maximum value is multiplied by a constant,
.alpha., equal to or smaller than 1, resulting in a product that
can be used as a threshold value. The center of gravity of an area
enclosed by a segment of a curve and a line representing the
threshold value is then determined, where the curve represents
variations in directional sharpness magnitude with the focal point
position and the segment represents points on the curve each having
a value greater than the threshold value. The center of gravity is
used as the center of the area under the curve representing the
directional sharpness magnitude.
That is, the center p.theta. of the area under the curve
representing the directional sharpness magnitude is found in
accordance with the following equation: ##EQU2##
FIG. 14 is a diagram showing a technique based on a weighted
average. If there are a plurality of maximum values for a
directional sharpness magnitude, peak positions of the maximum
values are found and a weighted average value of the peak positions
is computed with weights determined in accordance with the heights
of the maximum value points at the peak positions. The weighted
average is used as the center of the area under the curve
representing the directional sharpness magnitude. Let notations B
and C denote the positions of maximum values. In this case, the
center p.theta. of the area under the curve representing the
directional sharpness magnitude is computed in accordance with the
following equation: ##EQU3##
FIG. 15 is a diagram showing a technique based on symmetry
matching. In accordance with this technique, variations in degree
of matching with p.theta. are found. The degree of matching
represents coincidence between a curve d.theta.(f) representing
variations in directional sharpness magnitude with the focal point
position and a curve d.theta.(a-f) of image inversion symmetrical
with respect to an axis of symmetry f=a on the left and right sides
of the axis of symmetry f=a. The position a of an axis of symmetry
providing the highest degree of coincidence is determined as the in
focus position p.theta.. As a degree of coincidence, a point of a
maximum correlation value can also be used. As an alternative, a
point providing a minimum sum of squares of differences can also be
used as a degree of coincidence. Many other indicators generally
used as an indicator of the degree of coincidence can be used. The
embodiments described above are used for exemplifying a case in
which a charged particle beam apparatus is applied to an inspection
and measurement apparatus.
It should be noted, however, that the techniques described herein
with respect to the example of a charged particle beam apparatus
can also be applied to other equipment such as a fabrication
apparatus using a charged particle beam.
The preceding has been a description of the preferred embodiment of
the invention. It will be appreciated that deviations and
modifications can be made without departing from the scope of the
invention, which is defined by the appended claims.
The preceding has been a description of the preferred embodiment of
the invention. It will be appreciated that deviations and
modifications can be made without departing from the scope of the
invention, which is defined by the appended claims.
* * * * *